1. Field of the Invention
The present invention generally relates to a laser-induced plasma detecting system for use in a laser processing machine, and in particular to a laser control system utilizing a photo detector unit for detecting light of laser-induced plasma generated by application of laser beams on an object to be processed.
2. Description of the Prior Art
Conventionally, a laser processing machine has a torch unit 100 attached thereto for performing a laser beam machining process on an object to be processed as shown in FIG. 7. The torch unit is provided for the purpose of: i) obtaining a high energy density of laser beams required for processing the object by way of condensing the laser beams, and ii) obtaining assist gas flow to be projected onto a precise target position of the object.
In FIG. 7, the torch unit 100 includes a condenser lens 1 which is secured by a spacer ring member 2 disposed in close contact onto a spring ring member 3, where these members 1 through 3 are integrated and inserted in a lens holder tube 4, which is further secured by a lens securing ring 5. The lens holder tube 4 secured by the lens securing ring 5 is incorporated within a torch main body 6 of the torch unit 100 and securely fixed in position by a lens positioning ring 7.
The torch unit 100 further includes an assist gas inlet 15 and a nozzle attachment portion 101 which is attached to a torch end portion 8, where the nozzle attachment portion 101 is comprised of a nozzle 11 which is securely attached to a nozzle base portion 12, and a nozzle fixing member 9 which is securely positioned by a torch adjusting base plate 10 and fixed by a nozzle fixing ring 13 surrounding therearound.
In this construction, the nozzle base portion 12 is further mechanically engaged with the nozzle fixing member 9 by way of a screw cut means, where the positioning adjustment in a vertical axis direction of the nozzle is performed by adjusting the engagement between the nozzle fixing member 9 and the nozzle base portion 12 to thereby adjust the nozzle portion into a focus point for the laser beams.
In this arrangement, the nozzle attachment portion 101 is adjusted in position for centering alignment of the nozzle 11 by means of centering adjustment screw drivers 14 to obtain the alignment of the nozzle aperture with the laser beams passing therethrough. Thus, the torch unit 100 is adjusted in view of both the focus condition and the centering alignment with the laser beams.
In this construction of the conventional laser processing machine, particularly in a perforating process, the processing conditions are experientially selected in consideration of the conditions such as a pulse width, pulse period or frequency, processing time and processing speed recommended by makers without detecting laser-induced plasma (referred to as "LIP", hereinafter). It is noted here that the term "laser-induced plasma (LIP)" refers to a plasma phenomenon induced with evaporated gaseous materials of an object when applying laser beams onto the object to be processed. The LIP plays an important part of perforating, welding and like processes in laser beam machining.
However, in this conventional processing method, the laser beam machining process is not optimized with regard to the processing time because of employing the experientially decided conditions for the processing. In order to obtain the optimum conditions for performing the laser beam machining, not only elucidation but also detection of the process of the laser beam machining must be taken into account, and also it is required to judge the detection results of the processing phenomenon of the laser beam machining to control the laser beams based on the detection results.
In particular, when carrying out a perforating process on a mild steel plate having a thickness of, for example, 12 mm or more, it takes a long processing time to complete the perforation, and there arises a problem that a total processing time of the laser beam machining is undesirably increased especially when the laser beam machining process includes a lot of perforating steps.
FIGS. 8A and 8B shows a laser pulse signal for commanding generation of laser beams in connection with a detection signal of the laser-induced plasma LIP. In the conventional laser processing machine, as shown in FIGS. 8A and 8B, the detection signal is obtained by using a photo detector unit (not shown) for an experimental purpose to detect the intensity of the LIP although such a photo detector is not used in the practical conventional laser processing machine. In this experiment, a photoelectric photometer, light-flux meter and the like light-intensity meter may be used as the photo detector unit.
In this experiment, with regard to the conventional processing conditions experientially decided and recommended by makers, the pulse width of each pulse and the pulse period (i.e., pulse frequency) are both fixed. In a practical perforating process for a mild steel plate of 12 mm thick or more, whole processes are divided into two or three steps and the optimum conditions are decided in each divided step. However, the processing conditions are not optimized for each laser pulse in any process in the conventional method.
In this experiment, as shown in FIG. 8B, the peak values of the detection resultant signal are attenuated with time lapse. This is because the quantity of the detected light of the LIP is decreased as the perforation process proceeds in depthwise in the steel generated LIP is partially maintained in the perforated hole in the steel plate and the LIP part staying in the hole is not detected by the light detection unit.
In this experiment of the conventional process, there arise mainly three problems with regard to processing time under the processing conditions particularly in a perforating process by a laser processing machine.
As a first problem, each pulse period of the laser pulse signal is fixed longer than the period of life of each detection signal of the LIP, resulting in a drawback of increasing the redundant processing time as shown by a portion A in FIG. 8B.
Moreover, there is another problem that, as shown by a portion B in the figure, the processing conditions are not taken to cope with the deterioration in perforating property as the detection light quantity of the LIP is decreased with time lapse.
There is further another problem that, as shown by a portion C in FIG. 8A, the laser beam machining is still maintained for additional coverage for absorbing differences in process after completion of the perforation.